The present invention relates to the control of high-velocity fluid streams, such as those present in core and fan streams exhausting a gas turbine engine, and more particularly to the manipulation of such fluid streams through localized arc filament plasmas to affect noise radiation from and mixing rate in the mixing layers of these streams.
Noise radiation from an aircraft gas turbine engine is the dominant component of noise during takeoff and a major component during landing. As such, it is becoming an important issue for both commercial and military aircraft that are operating at considerably closer proximity to population centers, as there is mounting pressure to reduce noise propagated to adjacent communities. Commercial subsonic aircraft engine manufacturers have been able to satisfy increasingly stringent environmental noise regulations by using larger by-pass ratio engines. Unfortunately, the sheer physical size of current commercial subsonic aircraft engines is such that even larger bypass ratio engines are not practical. Additionally, in future supersonic commercial aircraft and also in high-performance military aircraft, large bypass ratio engines are not a viable option because of the performance penalties that such a design would incur.
It has been known for quite some time that large-scale coherent structures in jets are responsible for the entrainment and mixing of exhaust systems, and that their dynamical processes are responsible for a major portion of far field noise radiation. Research has indicated that these large-scale spanwise coherent structures in two-dimensional or ring-like coherent structures in axisymmetric jets or mixing layers, become more three-dimensional and less coherent as the compressibility level (which is generally proportional the ratio of velocity difference across the mixing layer to the average speed of sound in the two streams) is increased. This phenomenon renders these structures less amenable to control strategies similar to those traditionally used in incompressible and low Reynolds number flows. In contrast to these large-scale structures, longitudinal (streamwise) large-scale vortices do not seem to be much affected by compressibility. Thus, the use of streamwise vortices appears to be a logical approach in controlling mixing and consequently controlling the far field acoustic radiation in highly compressible jets.
In the past, several techniques have been explored in generating streamwise vortices. For example, small tabs or chevrons attached to the nozzle exit and used as streamwise vortex generators were found to be an effective device in enhancing mixing and altering noise characteristics in both incompressible and compressible jets, due to the presence of a spanwise pressure gradient set up in front of a tab, which since it protrudes into the flow, generates a spanwise pressure gradient regardless of whether the flow is subsonic or supersonic. In addition to streamwise vortices generated due to the spanwise pressure gradient, the streamwise pressure gradient generated by a tab promotes the development of robust spanwise vortices. Although the use of tabs and related protrusions to enhance mixing is effective in both incompressible and compressible flows, such use results in thrust losses due to the blockage effects. Gentler tabs, such as chevrons, can be used to reduce this thrust loss; however, their smaller profile necessitates weaker streamwise vortices and thus less mixing enhancement or noise alteration. Moreover, it is also beneficial to minimize the performance penalties associated with flowstream protrusions by having them deploy only during certain operational conditions (for example, during takeoff and landing in aircraft applications). Such an on-demand system requires complex tabs/chevrons geometries, ancillary actuation hardware and controllers, thereby exacerbating system complexity, weight and cost.
An alternative technique for generating streamwise vortices is the use of simple nozzle trailing edge modifications or cutouts. These cutouts are similar to chevrons, except that they do not protrude into the flow. Prior research has shown that such modifications have been effective in producing streamwise vortices that in turn generate enhanced mixing in incompressible axisymmetric jets, although the effectiveness of trailing edge modifications heavily depend on the flow regime. It was found that the use of trailing edge modifications enhanced mixing significantly in the underexpanded cases and moderately in the overexpanded cases. It was also found that the trailing edge modified nozzles substantially reduced the broadband shock associated noise radiation for both the underexpanded and overexpanded flow regimes, but did not significantly alter the noise field for the ideally expanded flow condition. While it is believed that the mechanism employed by nozzle trailing edge modifications to produce streamwise vortices is still a spanwise pressure gradient, it appears that such effects are relatively small in subsonic jets and heavily flow regime-dependent in supersonic jets. There is also evidence that trailing edge modifications exhibit a strong effect on the rate of jet mixing and thus noise radiation.
Another technique involves the use of fluidics, where pressurized fluid (typically air) is introduced into the flowpath to force an instability therein. Fluidic injection has not been entirely successful for use in high-speed flows for two main reasons. First, instability frequencies in high-speed flows are quite high, which necessitates that any actuation mechanism must possess high bandwidth capability. Second, fluid flows with high Reynolds numbers (such as those found in high subsonic and supersonic flow velocities) possess large dynamic loading within a noisy environment, which require high amplitude forcing. The lack of the availability of actuators with high bandwidth and high amplitude has been one of the main obstacles in fluidic control of high-speed flows. Efforts have been made to force shear layers in high Reynolds number at the jet column frequency; however, the required forcing amplitude is much higher than that used traditionally. Similarly, efforts have been made to develop high bandwidth and amplitude fluidic actuators. The main drawback of such fluidic actuators is the difficulty of establishing a reference time (or phase), for without such a reference time, the actuators cannot be used to force various azimuthal modes in axisymmetric jets. Since it is believed that certain of these azimuthal modes are instrumental in achieving noise reduction, the presence of an actuator that can excite such instabilities is highly desirable.
Still another technique that has been used in recent years exploits electric discharge plasmas for flow control. In a typical plasma-based approach, intense, localized and rapid heating is produced in the high-current pulsed electric discharges and pulsed optical discharges. This rapid near-adiabatic heating results in an abrupt pressure jump in the vicinity of the current-carrying filament. These pressure jumps in turn produce shock waves in supersonic flows, which can considerably modify the supersonic flow over blunt bodies and in supersonic inlets. Therefore, the rapidly heated regions act similar to physical geometry alterations (such as the tabs and trailing edge cutouts discussed earlier) in the flow but do so for short time durations. Various methods of plasma generation, including direct current (DC), alternating current (AC), radio frequency (RF), microwave, arc, corona, and spark electric discharges, as well as laser-induced breakdown, have been used to initiate plasma-based fields for flow control.
Previous investigations into plasma-based flow control have mainly focused on viscous drag reduction and control of boundary layer separation in low-speed flows, as well as shock wave modification and wave drag reduction in supersonic and hypersonic flows. In the previous high-speed research, spatially distributed heating induced by AC or RF glow discharges has been used to produce weak disturbances in the supersonic shear layer or to weaken the oblique shock in the supersonic inviscid core flow. These experiments have been conducted at fairly low static pressures, for example, at stagnation pressures of between 0.3 and 1.0 atmosphere with Mach numbers between two and four. This allowed initiating and sustaining diffuse glow discharges, which weakly affected relatively large areas of the flow. The main mechanism of the plasma flow control in these previous studies is heating of the flow by the plasma. In low-speed flows, the dominant plasma flow control mechanism is flow entrainment due to momentum transfer from high-speed directed motion of ions (i.e. electrical current) to neutral species (i.e. bulk flow) in the presence of a strong electric field. At these conditions, the ion velocity can be very high (for example, approximately 1000 meters per second for typical electric fields of 10 kilovolts per centimeter at one atmosphere). Although this approach was demonstrated to significantly vary the skin friction coefficient and to control the boundary layer separation in low-speed flows (at flow velocities up to a few meters per second), its applicability to high-speed flows is unlikely. The main disadvantage of this technique is that the ion number density in non-equilibrium plasmas is very low (for example, typical ionization fractions ni/N are between approximately 10−8 and 10−6), which limits the momentum transfer to the neutral species flow; such limited momentum transfer is not conducive for high flow velocities. Another disadvantage of this approach is the high power consumption of non-equilibrium plasmas (typically between approximately 10 to 100 watts per cubic centimeter), due to the fact that only a very small fraction of the total input power (often well below 1%) goes to direct momentum transfer from the charged species to the neutral species. The rest of the power (more than 90%) is spent on excitation of vibrational and electronic levels of molecules by electron impact, followed by flow heating during relaxation processes. This makes affecting large areas of the flow by such plasmas prohibitively expensive.
What is needed are actuators that can exploit streamwise vorticity generation, manipulation of jet instabilities, or a combination of the two techniques to facilitate noise reduction and flow mixing in high speed fluid flow environments. What is also needed are such actuators that can provide high amplitude, high bandwidth forcing while simultaneously being capable of withstanding harsh environments, such as those found in air-breathing turbomachinery and related power-generating equipment. What is additionally needed are actuators that do not interfere with fluid flow in the jet by protruding into the jet stream.